AC coupling is typically used in the input circuitry of test and measurement equipment to allow measurement of AC signals riding on top of DC signals. The AC coupling is usually located before any amplification stages so that amplifier gain switching does not disturb the quiescent state of the AC coupling capacitor. If gain were switched ahead of this capacitor, the DC value applied to the capacitor would also change and the circuit would require a relatively long period of time to settle. Additionally, applying the AC coupling early in the signal path allows removal of DC signals that could overload subsequent analog or digital stages.
A variety of AC coupling techniques are known. The simplest is to use a series capacitor between the input signal and the first amplification stage. Associated with the capacitor, however, is stray inductance, resistance, and nonlinearities due, e.g., to wire leads, and dielectric resistivity and absorption. In some applications, these stray effects can degrade circuit performance, such as by introducing RC time constants which limit the circuit's bandwidth or by introducing distortion components.
A second AC coupling technique relies on a DC buckout circuit. In a DC buckout circuit, a DC voltage is subtracted from the input signal, leaving just the AC component. Buckout AC coupling circuits, however, suffer from problems including drift of the subtraction voltage with temperature and other phenomena, and drift of the DC component of the input signal, both of which result in the coupled AC signal still having a DC component.
Some applications require that the circuit dynamically switch between AC and DC coupling while the circuit is in operation. Neither the coupling capacitor nor the DC buckout circuit can accommodate such switching without additional circuit components. The switching must be done at the AC coupling circuitry, prior to the amplification stage at the front end of the input circuitry because of the aforementioned gain switching problems. Input voltages at this location, however, can exceed the operational limits of active (e.g., semiconductor) switches, requiring relays to be used. But relays are relatively large, expensive and are prone to reliability problems. They can also exacerbate the problems of stray inductance and resistance noted earlier.
It is also difficult to maintain a constant input impedance in circuits capable of dynamically switching between AC and DC coupling. As those skilled in the art will recognize, a constant input impedance is important in many test/measurement applications.
Moreover, it is difficult to achieve sufficient initial accuracy and stability over time and environmental conditions of the AC coupling cutoff frequency, which is related to matching the phase response over multiple channels.
The present invention overcomes these and other limitations of the prior art. Some embodiments allow for dynamic switching between AC and DC coupling without the use of relays, while maintaining a constant input impedance. Drift of circuit parameters due to temperature, aging, or other phenomena have a reduced effect on performance in most embodiments.
In the preferred embodiment, a summing device combines an input signal with a subtraction signal to yield a DC-free output signal. Processing circuitry detects any DC component on the output signal and dynamically creates a corresponding subtraction signal to cancel the same.
In addition to overcoming the above-noted deficiencies, the preferred embodiment provides a number of further advantages as well. For example, the AC cutoff frequency can be dynamically changed, and the circuit's transfer function can be dynamically converted between single- and multiple-pole to change the AC passband shape.
The summing circuit can take a number of forms, including those based on resistive combiners and controllable current sources.
In a second embodiment, the circuit can control AC coupling filter response very accurately so that, for example, multiple channels remain phase matched at frequencies in the vicinity of the AC coupling cutoff frequency.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.